boron isotope fractionation during brucite deposition from artificial

$: Boron isotope fractionation during brucite deposition from artificial$
Clim. Past, 7, 693–706, 2011www.clim-past.net/7/693/2011/doi:10.5194/cp-7-693-2011© Author(s) 2011. CC Attribution 3.0 License.

Climateof the Past

Boron isotope fractionation during brucite depositionfrom artificial seawater

J. Xiao1, Y. K. Xiao2,3, C. Q. Liu3, and Z. D. Jin1

1State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences,Xi’an, 710075, China2Qinghai Institute of Salk Lakes, Chinese Academy of Sciences, Xining, Qinghai, 810008, China3State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences,Guiyang, 550002, China

Received: 21 February 2011 – Published in Clim. Past Discuss.: 2 March 2011Revised: 23 May 2011 – Accepted: 6 June 2011 – Published: 6 July 2011

Abstract. Experiments involving boron incorporation intobrucite (Mg(OH)2) from magnesium-free artificial seawaterwith pH values ranging from 9.5 to 13.0 were carried out tobetter understand the incorporation behavior of boron intobrucite and the influence of it on Mg/Ca-SST proxy andδ11B-pH proxy. The results show that both the concentrationof boron in deposited brucite ([B]d) and its boron partitioncoefficient (Kd) between deposited brucite and final seawa-ter are controlled by the pH of the solution. The incorpora-tion capacity of boron into brucite is almost the same as thatinto corals, but much stronger than that into oxides and clayminerals. The isotopic compositions of boron in depositedbrucite (δ11Bd) are higher than those in the associated arti-ficial seawater (δ11Bisw) with fractionation factors rangingbetween 1.0177 and 1.0569, resulting from the preferentialincorporation of B(OH)3 into brucite. Both boron adsorp-tions onto brucite and the precipitation reaction of H3BO3with brucite exist during deposition of brucite from artificialseawater. The simultaneous occurrence of both processes de-termines the boron concentration and isotopic fractionationof brucite. The isotopic fractionation behaviors and mech-anisms of boron incorporated into brucite are different fromthose into corals. The existence of brucite in corals can affecttheδ11B and Mg/Ca in corals and influences the Mg/Ca-SSTproxy andδ11B-pH proxy negatively. The relationship be-tweenδ11B and Mg/Ca in corals can be used to judge theexistence of brucite in corals, which should provide a reli-able method for better use ofδ11B and Mg/Ca in corals toreconstruct paleo-marine environment.

Correspondence to:Y. K. Xiao([email protected])

1 Introduction

Mg is a common trace element in corals and the Mg/Ca ratiohas been used to investigate the paleothermometry of seawa-ter (Watanabe et al., 2001; Mitsuguchi et al., 2008; Wei etal., 2009; Allison et al., 2010). The incorporation of Mg intocoral skeleton is controlled by varying factors. Mg can ex-ist in the form of brucite in corals (Smith and Delong, 1978;Nothdurft et al., 2005) and Mg content inscleractiniancoralsvaries between different genera and localities (Fallon et al.,1999). Nothdurft et al. (2005) studied brucite in livingscle-ractinian corals colonies (Acropora, Pocillopora, Porites)from subtidal and intertidal settings in the Great Barrier Reef,Australia, and subtidalMontastraeafrom the Florida Keys,United States. Their results suggested that high containingactivity combined with high pH and lowpCO2 led to local-ized brucite precipitation in organic biofilms of coral skele-tons and that brucite in living coral skeletons is an indicatorof extreme microenvironments in shallow-marine settings.The occurrence of brucite in corals can bring a higher Mg/Caratio and be responsible for reported anomalies in Mg/Cavs. sea-surface temperature (SST) plots in corals (Nothdurftet al., 2005). Defining the source of such anomalies is criticalfor using Mg/Ca in corals to reconstruct the paleothermome-try of seawater (Mg/Ca-SST proxy).

In recent years, topics such as the reconstruction of an-cient seawater pH using the isotopic composition of boron incorals, the calculation of the pastpCO2, and the influenceof these two factors on changes in the ancient climate, havebecome important issues for the international boron isotopegeochemistry community (δ11B-pH proxy) (Hemming andHanson, 1992; Gaillardet and Allegre, 1995; Pelejero et al.,2005; Liu et al., 2009; Wei et al., 2009; Douville et al., 2010).

Published by Copernicus Publications on behalf of the European Geosciences Union.

http://creativecommons.org/licenses/by/3.0/

694 J. Xiao et al.: Boron isotope fractionation during brucite deposition from artificial seawater

Table 1. Chemical composition of magnesium-free, artificial seawater.

Chemical composition NaCl CaCl2 NaHCO3 KCl NaBr H3BO3 Na2SiO3 Na2Si4O9 H3PO4 Al2Cl6 LiNO3

Concentration (g l−1) 26.7260 1.1530 0.1980 0.7210 0.0580 0.2612 0.0024 0.0015 0.0020 0.0130 0.0013

Whether theδ11B of corals is equal to that of B(OH)−

4 hasemerged as one of the main questions involved in these is-sues. A series of inorganic calcite precipitation experimentshave shown that B(OH)−

4 is the only or dominant species in-corporated into calcite (Hemming et al., 1995; Sanyal et al.,1996, 2000). However, Klochko et al. (2009) recently foundthat both trigonal and tetrahedral coordinated boron existedin biogenic and hydrothermal carbonates. Rollion-Bard etal. (2011) also found both boron coordination species, but indifferent proportions depending on the coral microstructure,i.e. centres of calcification versus fibres. They suggested thatcareful sampling is necessary before performing boron iso-topic measurements in deep-sea corals (Rollion-Bard et al.,2011). Inorganic calcite precipitation experiment has beencarried out by Xiao et al. (2006a), indicating that theδ11B ofinorganic calcium carbonate was not parallel with the cal-culated curve of B(OH)−4 , but deviated increasingly fromthe parallel trend as pH increased. When the pH was in-creased to a certain value, the isotopic fractionation factor ofboron between precipitation and solution was greater than 1.Xiao et al. (2006a) reasoned that the presence of Mg2+ orother microelements was the main reason for this observa-tion and concluded that B(OH)3 incorporated preferentiallyinto brucite. If this is true, the isotopic compositions of boronin corals can be affected by the existence of brucite in coralsand the preferential incorporation of B(OH)3 into brucite. Ifthe brucite-bearing corals were used in theδ11B-pH proxy,the measuredδ11B of corals and the calculated pH will behigher than the normal value, which influences theδ11B-pHproxy negatively. However, little attention has been paid tothe mechanism of boron incorporated into brucite and its in-fluence on the boron isotopic composition of corals.

In this study, experiments on the incorporation of boronduring the deposition of brucite from magnesium-free arti-ficial seawater at various pH values were carried out. Theincorporation species of boron into brucite and the the boronisotope fractionation during deposition of brucite were de-termined. The influence of brucite in corals on Mg/Ca andtheδ11B, judgment method for existence of brucite in coralsand its potential application in theδ11B-pH proxy and theMg/Ca-SST proxy were discussed. This result will shedsome light on the application of the Mg/Ca-SST proxy andtheδ11B-pH proxy.

2 Methods

2.1 Reagents and equipment

1.0 M HCl, 0.5 M NaOH, and 0.5 M MgCl2 were preparedusing boron-free water with twice distilled HCl (GR), NaOH(GR), and MgCl2 (GR), respectively. The concentration ofthe Cs2CO3 (99.994 % purity) solution used in our study was12.3 g Cs l−1. A suspended graphite solution was preparedusing spectrum pure (SP) graphite mixed with 80 % ethanoland 20 % water. For a mannitol concentration of 1.82 g l−1,GR mannitol mixed with boron-free water was used. Theboron-free water was obtained by using 18.2 M� milliQ ofwater exchanged with an Amberlite IRA 743 boron specificresin. A GV IsoProbe T-single magnetic sector thermal ion-ization mass spectrometer was used for boron isotope analy-sis. A 2100 type and UV-2100 type spectrophotometer fromShanghai Unico Ltd was used for boron concentration anal-ysis. A D/Max-2200 type X-ray diffraction equipment fromJapan neo-confucianism company was used for XRD anal-ysis. A TP310 type pH meter from Beijing Time PowerCo. LTD was used for pH analysis.

Throughout all the experiments, vessels of Teflon,polyethylene or chert were used for avoiding boron contam-ination.

2.2 Magnesium-free artificial seawater

Magnesium-free artificial seawater was prepared by Mo-cledon prescription. The chemical composition of themagnesium-free artificial seawater is given in Table 1. Inorder to get sufficient quantities of boron for precise isotopicanalysis in the precipitated Mg(OH)2, boron concentrationin the artificial seawater was adjusted to 45.9 ppm, approx-imately 10 times that of normal seawater (∼4.5 ppm). Ac-cording to the result reported by Ingri et al. (1957), therewas no polyborate species present when the boron concen-tration of aqueous solution was 45.9 ppm. The pH valueand boron isotopic composition of the initial artificial sea-water was 3.64 and−7.00± 0.07 ‰, respectively. Sincemagnesium-free artificial seawater was used in our experi-ments, no brucite was deposited while the pH was being ad-justed. Brucite was deposited only when MgCl2 was addedafter the pH adjustments were completed.

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J. Xiao et al.: Boron isotope fractionation during brucite deposition from artificial seawater 695

2.3 Experimental deposition of brucite

Deposition experiments were carried out in a pH-controlledsystem at a temperature of 22± 0.5◦C in a super-clean lab-oratory (100 Class). Firstly, about 200 ml of artificial seawa-ter was placed inside a plastic beaker. Solutions of NaOHwere slowly added to each beaker to achieve the followingrange of pH values: 9.5, 10.0, 10.5, 11.0, 11.5, 12.0, 12.5,and 13. 0. 8 ml of MgCl2 solution was then added dropwiseinto the artificial seawater with different pH values. Duringthis process, NaOH was added to keep the pH at the prede-termined pH values as the brucite was deposited. The pHfluctuations were measured by a pH meter with a precisionof 0.02. A magnetic stirrer was used during the experimentalprocess to ensure the reactions were completed. The exper-iment was terminated after 8 ml of MgCl2 was added. Aftermixing the deposit and solution well, the mixture was di-vided into two parts equally. One was filtrated by pumpingimmediately to isolate deposit from the solution (defined as arepose timeTr = 0 h) and the other was filtrated by pumpingafter reposing for 20 h (defined as a repose timeTr = 20 h).The separated solution was placed in air-tight plastic bottles,and the brucite deposit was washed with boron-free water un-til Cl− was no longer detected. Then the deposited materialwas dried at 60◦C and placed in air-tight plastic bottles.

2.4 Separation of boron from the samples

In preparation for isotopic measurement, boron was extractedfrom the artificial seawater and the brucite using a two-step chromatographic technique (Wang et al., 2002). About0.5 ml of Amberlite IRA 743 boron-specific resin with 80–100 mesh was placed in a polyethylene column with a diam-eter of 0.2 cm. The length of the resin bed was 1.5 cm. About200 mg of brucite was first dissolved using a slightly exces-sive amount of boron-free HCl solution, passed through acolumn filled with Amberlite IRA 743 resin, and then elutedusing approximately 5 ml of 0.1 M HCl at 75◦C. An amountof mannitol approximately equimolar to the boron contentwas added to the eluate, which was then desiccated to neardryness by partial evaporation in a super clean oven withlaminar flow at 60◦C (Xiao et al., 2003). The solution wasthen loaded again into the column filled with a mixed resincomposed of 0.5 ml cation-exchange resin (H+ form) and0.5 ml anion-exchange resin (ion- exchange II, HCO−

3 form).The boron was eluted by 5 ml of boron-free water. The elu-ate was evaporated again at 60◦C to produce a solution witha boron concentration of about 1 µg B µl−1 for isotopic mea-surement. The procedures for extracting boron in the liquidsamples were the same as those described above. The totalboron blank in chemical process and filament loading wasdetermined by isotope dilution mass spectrometry (IDMS)to contain 46 ng of boron.

2.5 Measurements of boron concentration and isotopiccomposition

The boron concentrations were determined by theazomethine-H spectrophotometric method. One ml ofsample solution, 2 ml of buffer solution, and 2 ml ofazomethine-H solution were added in that order. Afterthorough mixing, each solution was allowed to stand for120 min. The absorption of the boron-azomethine-H com-plex was measured at 420 nm using a spectrophotometer.The external precision of the azomethine-H method usedhere was 2 %.

The isotopic compositions of boron in all samples weremeasured by a GV IsoProbe T single magnetic sector thermalionization mass spectrometer and the P-TIMS method usingCs2BO+

2 ions with a graphite loading (Xiao et al., 1988).Two µl of graphite slurry was first loaded onto a degassedtantalum filament, and then approximately 1 µl of sample so-lution containing about 1–4 µg of boron and an equimolaramount of mannitol were also loaded. This was followed by1–2 µl of a Cs2CO3 solution containing an equimolar amountof cesium. The loaded material was dried by heating the fil-ament at 1.2 A for 5 min.

The data were collected by switching the magneticfield between the masses 308 (133Cs10

2 B16O+

2 ) and 309(133Cs11

2 B16O+

2 ), and the intensity ratios of the ion beams atmasses 308 and 309 (R309/308) were calculated. Calibratedwith oxygen isotopes, the11B/10B ratio was calculated asR309/308−0.00078. The isotopic composition of boron wasexpressed as anδ11B value according to the following for-mula:

δ11B (‰) =

[(11B

/10B

)sample

/(11B

/10B

)standard

− 1

]× 1000

Here the standard material is NIST SRM 951, the rec-ommended value of which is 4.04362± 0.00137 (Catan-zaro et al., 1970). The measured averageδ11B/10B ratio ofNIST SRM 951 was 4.0525± 0.0027 (2σm, n = 5).

3 Results and discussion

3.1 Dissolved boron species and boron isotope inseawater

The dominant aqueous species of boron in seawater areB(OH)3 and B(OH)−4 . The relative proportions of thesespecies are a function of pH (Fig. 1a) and given by the fol-lowing relation:

B(OH)3 + H2O ↔ B(OH)−4 + H+ (1)

Figure 1 shows the theoretical distribution of boron speciesas a function of pH for seawater. At low pH (pH< 7), vir-tually all of the boron in seawater is in the B(OH)3 species;conversely, at high pH (pH> 10), virtually all of the boron

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7 8 9 10 11 12 13

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ion

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oron

spe

cies

(a)

[B(OH)3]

[B(OH)4-]

7 8 9 10 11 12 1

pH3

20

30

40

50

60

δ11B

(0/00

)

(b)

B(OH)4-

B(OH)3

modern seawater

Fig. 1. Fraction (a) of the major dissolved boron species B(OH)3 (green curve) and B(OH)4

– (red curve) in seawater, using pKb=8.597 (Dickson, 1990) and stable boron isotope fractionation (b) between them as a function of pH, using α4-3=0.974 (Klochko et al., 2006). At low pH (pH<7), virtually all of the boron in seawater is in the B(OH)3 species; conversely, at high pH (pH>10), virtually all of the boron is in the B(OH)4

- species (Fig. 1a). Only the B(OH)4− incorporated into the marine bio-carbonates is one of the

essential hypotheses for using boron isotopic composition in marine bio-carbonate to reconstruct the paleo-environment (Hemming and Hanson, 1992).

19

Fig. 1. Fraction(a) of the major dissolved boron species B(OH)3 (green curve) and B(OH)−

4 (red curve) in seawater, usingpKb = 8.597(Dickson, 1990) and stable boron isotope fractionation(b) between them as a function of pH, usingα4−3 = 0.974 (Klochko et al., 2006).At low pH (pH< 7), virtually all of the boron in seawater is in the B(OH)3 species; conversely, at high pH (pH> 10), virtually all of theboron is in the B(OH)−4 species (Fig. 1a). Only the B(OH)−

4 incorporated into the marine bio-carbonates is one of the essential hypothesesfor using boron isotopic composition in marine bio-carbonate to reconstruct the paleo-environment (Hemming and Hanson, 1992).

is in the B(OH)−4 species (Fig. 1a). The isotope exchangereaction between these species is given by:

10B(OH)3 +11B(OH)−4 ↔

11B(OH)3 +10B(OH)−4 (2)

The distribution of species and the isotope exchange re-action are solved simultaneously to give the isotopic com-position of the individual boron species vs. pH for seawa-ter (Fig. 1b). The abundances of two stable isotopes11Band10B at pH 8 make up approximately 80 % and 20 % ofthe total boron, respectively. Modern seawater has a boronconcentration of about 4.5 ppm, and a consistent, world-wide δ11B isotopic composition of +39.61± 0.04‰ relativeto NIST SRM 951 (Foster et al., 2010). The isotopic compo-sition of B(OH)−4 increases with pH and so does theδ11B ofcarbonates, provided that B(OH)−

4 is preferentially incorpo-rated in the carbonates. As a result, variations of seawater pHin the past should be traceable by variations ofδ11B in fos-sil carbonates. This is the basis of the boron paleo-pH proxy(Hemming and Hanson, 1992).

3.2 The partition coefficient of boron between Bruciteand final seawater

The boron concentrations and the partition coefficients (Kd)between brucite and final seawater are listed in Table 2, andshown in Fig. 2. The results are shown as follows:

1. The variation trend of the results forTr = 0 h andTr = 20 h are basically the same (Fig. 2a), indicating theincorporation of boron into brucite deposit is instan-taneous, and the reaction time is not a main limiting

factor. This finding is consistent with the previous study(Liu et al., 2004).

2. When the pH is lower than 10, [B]fsw decreases as thepH increases, and reaches the lowest value at pH 10(Fig. 2a). This indicates that the amount of boron in-corporated into deposited brucite increases as the pH in-creases, and reaches the highest value at pH 10. [B]fswincreases quickly at higher pH values, and no smoothtrend is observed, indicating that the amount of incor-porated boron decreases quickly.

3. When pH is lower than 10,Kd increases as the pH in-creases (Fig. 2b). TheKd reaches its highest values of487.5 and 494.2 forTr = 0 h andTr = 20 h at pH 10, re-spectively. When the pH is higher than 10,Kd decreasesand shows a mild trend after pH 12.

TheKd between brucite deposit and final seawater can beshown as:

Kd = (f × Kd3 + Kd4)/(1 + f ) (3)

Kd = {[B3]solid + [B4]solid}/{[B3]fluid + [B4]fluid},Kd3 = [B3]solid/[B3]fluid, Kd4 = [B4]solid/[B4]fluid,f = [B3]fluid/[B4]fluid, Kd3 and Kd4 are the partition co-efficient of B(OH)3 and B(OH)−4 , respectively.f3 is the ratioof B(OH)3 to B(OH)−4 in solution. This equation indicatesthat Kd is not only related withKd3 and Kd4, but alsowith the fraction of B(OH)3 in solution. TheKd betweenclay minerals and seawater is 3.60 atT = 5◦C and pH 8.45(Palmer et al., 1987), but the highestKd in our experimentsis 494 at pH 10, which is much higher than 3.6. It indicates



Table 2. Boron concentration (ppm) and partition coefficientKd between brucite deposit and final seawater.

Trepose pH 9.5 10.0 10.5 11.0 11.5 12.0 12.5 13.0

0 h [B]fsw 4.60 1.92 2.59 4.63 10.3 15.6 21.7 22.9[B]d 901 938 845 626 658 290 274 250Kd 196 488 326 135 64.2 18.6 12.6 10.9

20 h [B]fsw 5.16 1.93 3.17 5.80 15.3 20.8 21.7 24.6[B]d 1859 952 885 844 685 413 326 229Kd 360 494 279 146 44.8 19.9 15.0 9.31

[B]fsw and[B]d are the boron concentrations in final seawater and deposit, respectively.

9.0 10.0 11.0 12.0 13.0

pH

0

5

10

15

20

25

[B] fs

w (p

pm)

9.0 10.0 11.0 12.0 13.0

pH

0

100

200

300

400

500

Kd

(b)

0

400

800

1200

1600

2000,Solid line: Tr=0h+B +Broroken line: Tr=20h

(a)

[B] d (

ppm

)

final seawater

deposit

,Solid line: Tr=0hken line: Tr=20h

Fig. 2. Boron concentrations (a) and partition coefficient Kd (b) between brucite deposit and final seawater for Tr=0 h and 20 h. The variation trend of the results for Tr = 0 h and Tr = 20 h are basically the same (Fig. 2a), indicating the incorporation of boron into brucite deposit is instantaneous. [B]fsw decreases firstly and then increases quickly with the lowest value at pH 10 (Fig. 2a), indicating the amount of boron incorporated into deposited brucite increases firstly and decrease as pH increases, and reaches the highest value at pH 10. The variation trend of Kd is contrary to that of [B]fsw (Fig. 2b).

20

Fig. 2. Boron concentrations(a) and partition coefficientKd (b) between brucite deposit and final seawater forTr = 0 h and 20 h. Thevariation trend of the results forTr = 0 h andTr = 20 h are basically the same (Fig. 2a), indicating that the incorporation of boron into brucitedeposit is instantaneous. [B]fsw decreases firstly and then increases quickly with the lowest value at pH 10 (Fig. 2a), indicating the amountof boron incorporated into deposited brucite increases firstly and decrease as pH increases, and reaches the highest value at pH 10. Thevariation trend ofKd is contrary to that of [B]fsw (Fig. 2b).

that theKd3 is much greater thanKd4, i.e. the incorporationcapacity of B(OH)3 into brucite is much higher than thatof B(OH)−4 and theKd3 contribution is dominating. Thequantity of brucite reaches its highest at pH 10, indicatingthe reaction equivalence point of brucite deposition occursapproximately at pH 10. TheKd also reaches its highest atpH 10. When pH is higher than 10, thef andKd3 decreaseswhile Kd4 increases as the increasing pH. Although the in-creasing concentration of OH− in solution can influence theadsorption of B(OH)−4 (Keren et al., 1981), the adsorptionof B(OH)−4 continues, and theKd4 still increases perhaps.Because ofKd3 is greater thanKd4, the decreasing ofKd3 isdominating, so that theKd decreases with the increasing pH.In the study of Xu and Ye (1997), the point of zero charge(PZC) of brucite is close to 11.9, which approximates to theinflexion point (pH 12) in our experiment. When the pH ishigher than PZC, anions will not be adsorbed by goethiteand gibbsite (Kingston et al., 1972), so that the adsorption

ability of B(OH)−4 by brucite decreases greatly when thepH is higher than the PZC of brucite. Therefore,Kd shouldmonotonically decrease with increasing pH as well.

Boron removal experiments by magnesium hydroxide atdifferent pH values have been carried out by Liu et al. (2004),de la Fuente and Munoz (2006) and Yuan et al. (2006). Theyattributed the adsorption of B(OH)−

4 to the magnesium hy-droxide. Their results showed that [B]fsw reached its lowestvalues at pH 9.3 or 10, which indicates that adsorption hadreached its highest values. The [B]fsw then increased slowlyat higher pH values. The results of the present study are con-sistent with their work.

There is a slight difference in the variation of [B]d withpH for differentTr values. WhenTr = 0 h and pH = 9.5, [B]dis slightly lower than that at pH 10, and the highest [B]d valueof 937.8 ppm also appears at pH 10. But whenTr = 20 h,the maximum [B]d may appear at pH≤ 9.5, while the high-est value at pH 9.5 is 1859 ppm, which is much higher than951.5 ppm at pH 10. Considering the slight difference in the



0.9680

0.9720

0.9760

0.9800

0.9840

0.9880

Isot

opic

frac

tiona

tion

fact

or

9.0 10.0 11.0 12.0 13.0

pH

-40

-20

0

20

40

δ11B

(0/00

)

,Solid line: Tr=0h+Broken line: Tr=20h

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deposite

final seawater

(a)

9.0 10.0 11.0 12.0 13.0

pH

1.0100

1.0200

1.0300

1.0400

1.0500

1.0600

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opic

frac

tiona

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fact

or

,Solid line: Tr=0hroken line: Tr=20h

(b)+B

,Solid line: αcarb-3

roken line: αcarb-sw

(c)+B

7.8 8.0 8.2 8.4 8.6

pH

Fig. 3. Isotopic compositions (a) and fractionation factor (αd-fsw) (b) of boron between brucite deposit and solution for Tr= 0 h and 20 h and (c) between inorganic carbonate and seawater (Sanyal et al., 1996). The variation trend of δ11B for Tr = 0 h and Tr = 20 h are basically same and all the δ11Bd values are enriched in the heavy isotope relative to the δ11Bfsw (Fig. 3a). All the αd-fsw are greater than 1 and shows a waved pattern (Fig. 3b), which is entirely different from that of the carbonates deposits (Fig. 3c).

21

Fig. 3. Isotopic compositions(a) and fractionation factor (αd−fsw) (b) of boron between brucite deposit and solution forTr = 0 h and 20 hand(c) between inorganic carbonate and seawater (Sanyal et al., 1996). The variation trend ofδ11B for Tr = 0 h andTr = 20 h are basicallysame and all theδ11Bd values are enriched in the heavy isotope relative to theδ11Bfsw (a). All the αd−fsw are greater than 1 and shows awaved pattern(b), which is entirely different from that of the carbonates deposits(c).

boron balance time of the deposited Mg(OH)2, the value forTr = 20 h may be more reliable.

The evolution pattern of boron incorporated into theMg(OH)2, and the variations in partition coefficients withthe pH, are basically the same as those of oxides and clayminerals. However, the maximum amounts of incorporatedboron in brucite or other hydroxides are much higher thanthose of the oxides and clay minerals (Keren et al., 1981;Keren and Gast, 1983; Goldberg and Glaubig, 1985, 1986;Lemarchand et al., 2005, 2007), indicating that the incor-poration capacity of boron on hydroxides is stronger thanthat of both oxides and clay minerals. The boron concen-tration in corals varies from 39 to 117.5 ppm (Vengosh et al.,1991; Gaillardet and Allegre, 1995; Xiao et al., 2006b; Liuet al., 2009). If 4.5 ppm of the seawater boron concentrationis used, theKd between coral and seawater varies from 8.7 to26.1. In our experiments, the boron concentration in the ini-tial artificial seawater is 45.9 ppm, finally the level of boronconcentration in brucite depositions can be divided by 10. Inthis case, the expected boron content would range between22 and 95 ppm (excluding the aberrant value of 185.9 ppmobtained at pH 9.5), so theKd between brucite and final sea-water varies from 4.8 to 19.2. Such a range of boron con-centrations andKd is of the same order of magnitude as thecommon boron contents measured in corals.

3.3 Isotopic fractionation of boron between depositedbrucite and solution

Isotopic compositions of boron in deposited brucite (δ11Bd)and final seawater (δ11Bfsw) are listed in Table 3 and shownin Fig. 3. The crucial points illustrated in Fig. 3 are summa-rized as follows:

1. The variation trend of the results forTr = 0 h andTr = 20 h are basically same (Fig. 3a). This indicates

that the isotopic balance between brucite deposit andsolution is instantaneous, and that the results of the ex-periment are exact. A minor difference betweenTr = 0 handTr = 20 h appears as the pH increases from 9.5 to10.0. WhenTr = 0 h, δ11Bd and αd−fsw both increaseas the pH increases from 9.5 to 10.0. However, whenTr = 20 h,δ11Bd andαd−fsw both decrease as the pH in-creases from 9.5 to 10.0. This is perhaps related to thefact that the formation of brucite is slower at low pHthan at high pH.

2. All the δ11Bd values are enriched in the heavy isotoperelative to theδ11Bfsw (Fig. 3a). All theαd−fsw aregreater than 1 (Fig. 3b), which is entirely different fromthose of the carbonates deposits (Fig. 3c). As B(OH)−

4 isincorporated preferentially into carbonate deposits, alltheδ11B values of carbonates are lower than that of theparent seawater. Meanwhile, all the isotopic fractiona-tion factorsαcarb−sw between carbonates and seawaterare lower than 1 and increase with pH. In this study,theαd−fsw between brucite deposit and solution showsa waved pattern (Fig. 3b), suggesting a complex isotopicfractionation process.

The δ11Bd, δ11Bisw3 and δ11Bisw4 of B(OH)3 andB(OH)−4 in initial seawater, calculated using the dif-ferent α4−3 versus pH values of the parent solutions,are shown in Fig. 4. WhenTr = 0 h, theδ11Bd val-ues are equal to or lower than theδ11Bisw3 calcu-lated usingα4−3 = 0.9600. WhenTr = 20 h, theδ11Bdvalues are equal to or lower than theδ11Bisw3 calcu-lated usingα4−3 = 0.9397, but all theδ11Bd values arehigher thanδ11Bisw (Fig. 4). Theα4−3 of 0.9600 and0.9397 is calculated according to the highestδ11B ofdeposited brucite forTr = 0 h andTr = 20 h. If δ11Bdis equal toδ11Bisw3, it implies that only B(OH)3 is



Table 3. Isotopic fractionation of boron between deposited brucite and solution.

Repose time 0 h 20 h

pH δ11Bd (‰) δ11Bfsw (‰) αbd−fsw δ11Bd (‰) δ11Bfsw (‰) αb

d−fsw

9.5 3.54± 0.1(2)a −25.09± 0.1(2) 1.0294 31.27(1) −24.27(1) 1.056910.0 7.77± 0.5(2) −31.11± 0.1(2) 1.0401 8.57(1) −29.53(1) 1.039310.5 1.89± 0.1(2) −29.38± 0.2(2) 1.0322 −1.20± 0.3(2) −31.85(1) 1.031711.0 2.61± 0.1(2) −27.33± 0.5(2) 1.0308 5.04(1) −29.87(1) 1.036011.5 2.21± 0.3(3) −27.38± 0.5(3) 1.0304 2.45(1) −18.15± 0.5(2) 1.021012.0 8.67± 0.1(2) −11.23± 0.3(2) 1.0201 8.99(1) −8.55(1) 1.017712.5 17.53± 0.1(2) −12.86± 0.6(2) 1.0308 18.89(1) −14.92(1) 1.034313.0 27.84± 0.2(3) −8.58± 0.7(2) 1.0367 28.26± 0.4(2) −10.63(1) 1.0393

aThe number in brackets indicate measurement times;bαd−fsw is the isotopic fractionation factor of boron between deposit and final seawater, calculated asαd−fsw = (1000 +δ11Bd)/(1000 +δ11Bfsw).

6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0

pH

-80

-60

-40

-20

0

20

40

60

δ11Β

(0/00

)

δ11Βisw4, α4−3=0.9397

δ11Βisw4, α4−3=0.9600

δ11Bisw3, α4-3=0.9397

δ11Bisw3, α4-3=0.9600

δ11Βisw

, δ11Βd, Tr=0h+ δ11Βd, Tr=20h

Fig. 4. δ11B of brucite deposit (δ11Bd), and δ11B of B(OH)3 (δ11BBisw3) and B(OH)4

- (δ B11isw4) in initial

seawater calculated using different α4-3 versus pH values of parent solution. When Tr = 0 h, the δ Bd 11

B values (black solid circle) are equal to or lower than the δ11Bisw3 calculated using α4-3 = 0.9600 (black line). When Tr = 20 h, the δ11Bd values (red circle)are equal to or lower than the δ11Bisw3 calculated using α4-3 = 0.9397 (green line), but all the δ11Bd values are higher than δ11BBisw (blue line).

22

Fig. 4. δ11B of brucite deposit (δ11Bd), and δ11B of B(OH)3(δ11Bisw3) and B(OH)−4 (δ11Bisw4) in initial seawater calculatedusing differentα4−3 versus pH values of the parent solution. WhenTr = 0 h, theδ11Bd values (black solid circle) are equal to or lowerthan the δ11Bisw3 calculated usingα4−3 = 0.9600 (black line).WhenTr = 20 h, theδ11Bd values (red circle)are equal to or lowerthan theδ11Bisw3 calculated usingα4−3 = 0.9397 (green line), butall theδ11Bd values are higher thanδ11Bisw (blue line).

incorporated into Mg(OH)2, which is impossible undernormal circumstances. Moreover a situation in whichδ11Bd > δ11Bisw3 would be even more improbable. Ex-cept for pH 9.5,δ11Bd values are lower thanδ11Bisw3calculated usingα4−3 = 0.9600 (Fig. 4), so that the trueα4−3 would have to be lower than 0.9600, which is alittle lower than the currently expected value between

0.983 (Sanchez-Valle et al., 2005) and 0.952 (Zeebe,2005). The pH values used in previous carbonate pre-cipitation experiments are lower than 9.0, while that ofour experiments are between 9.5 and 13.0. The differ-ences of the experiments conditions may result diverseα4−3, but which value is much more reliable or the trueα4−3 is still uncertain. Our results also indicate thatδ11Bd is not parallel withδ11Bisw3, but first decreasesand then increases with an inflexion at pH 12. All of thedata indicate that both B(OH)3 and B(OH)−4 are incor-porated into the deposited brucite.

The fractions of B(OH)3(Fd3) in the brucite depositcalculated usingα4−3 = 0.9397 andα4−3 = 0.9600 areshown in Fig. 5. It seems thatFd3 tends to decreasewith decreasingα4−3. Because the adoptedα4−3 is nottrue, the calculatedFd3 is not exact. The trueFd3 couldbe lower than the value indicated in Fig. 5, but the vari-ation trend ofFd3 with pH should be certain and muchhigher than that of the initial artificial seawater (Fisw3).Theδ11Bd is the concurrent result ofδ11Bd3 andδ11Bd4.Hereδ11Bd can be shown as:

δ11Bd = fd3 × δ11Bd3 + (1 − fd3) × δ11Bd4 (4)

wherefd3 is the fraction of B(OH)3 in solid. DuringpH 10–12, bothδ11Bd3 and δ11Bd4 increases with in-creasing pH (Fig. 1b). Therefore, theδ11Bd shouldbe increased with the increasing pH, but the increas-ing trend is inconspicuous (Fig. 3a). There are two rea-sons accounting for this. On the one hand, the increas-ing δ11Bd3 and δ11Bd4 can result in the increasing ofδ11Bd. On the other hand, with the increasing pH, thedominant decreasing incorporation fraction of B(OH)3and increasing incorporation fraction of B(OH)−

4 willresult the deceasing ofδ11Bd. With the common influ-ence of these two aspects, the variation ofδ11Bd withpH is small. When the pH reaches 12 (PZC of brucite),



9 10 11 12 1

pH3

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Frac

tion

of B

(OH

) 3

-,- Tr = 0h, α4−3=0.9660-&- Tr = 0h, α4−3=0.9397-$- Tr=20h, α4−3=0.9660-+- Tr=20h, α4−3=0.9397

Fd3 in final seawater

Fig. 5. Fraction Fd3 of B(OH)3 in brucite deposit and final seawater calculated according to δ11Bd with different α4-3. The variation trend of Fd3 for different repose time is basically same. Fd3 tends to decrease with decreasing α4-3 and is much higher than that of the initial artificial seawater (blue curve).

23

Fig. 5. FractionFd3 of B(OH)3 in brucite deposit and final seawatercalculated according toδ11Bd with different α4−3. The variationtrend ofFd3 for a different repose time is basically the same.Fd3tends to decrease with decreasingα4−3 and is much higher than thatof the initial artificial seawater (blue curve).

the adsorption quantity of B(OH)−

4 decreases suddenly,while that of B(OH)3 increases (Fig. 5). So theδ11Bdincreases.

3. The manner in whichαd−fsw changes with pH forTr = 0 h and 20 h is shown in Fig. 3b. Except for pH 9.5,both patterns are basically the same for eachTr, andmatch the change inFd3 shown in Fig. 5. Theαd−fswvalues decrease as the pH increases from 9.5 to 12, andreaches the lowest point at pH 12. Thenαd−fsw valuesincrease rapidly. Theαd−fsw reflects the relative ratio ofB(OH)3 to B(OH)−4 in brucite.

The explanation ofδ11Bd with pH can be also usedfor the explanation of the boron fractionation factorbetween brucite and solution. When the pH rangesfrom 10 to 12, the incorporation fraction of B(OH)3 de-creases continuously (Fig. 5), which is contrary to thatof B(OH)−4 . This lead to the deceases ratio of B(OH)3 toB(OH)−4 , so theαd−sw decreases. When pH reaches 12(PZC of Mg(OH)2), the adsorption quantity of B(OH)−

4decreases suddenly, and the relative ratio of B(OH)3 toB(OH)−4 increases (Fig. 5), so theαd−fsw also increases.

Our results indicate that B(OH)3 is preferentially incor-porated into the solid phase during the deposition ofbrucite. In a precipitated experiment of calcium car-bonates from artificial seawater, Xiao et al. (2006a) ob-tained the result that the boron isotopic fractionation

factors between calcium carbonates and seawaters werehigher than 1. They reasoned that the presence ofMg2+ or other microelements was the main reason forthis observation and concluded that the heavier B(OH)3species was incorporated preferentially into Mg(OH)2.The results of our study have further confirmed the con-clusion of Xiao et al. (2006a).

4 Model of boron incorporated into brucite

4.1 Model 1: boron adsorption by minerals

The boron adsorption by minerals can be explained by phe-nomenological equation (Keren et al., 1981). It is supposedthat when B(OH)3, B(OH)−4 and OH− coexist in solution,they can be adsorbed by Mg(OH)2, metal oxides and clayminerals, and the affinity coefficientsKB3, KB4 andKOH areKB3 < KB4 < KOH for B(OH)3, B(OH)−4 and OH−, respec-tively (Keren et al., 1981). They compete for the same ad-sorption site on solid. When the pH is less than 7, B(OH)3isdominant. BecauseKB3 to clay mineral is very low, theboron adsorption is relatively minor. Under this pH value,the concentration of B(OH)−

4 and OH− is very low. Al-though they have a relatively stronger affinity to clay, theircontribution to the total boron adsorption is still very small.When the pH increases to 9, B(OH)−

4 concentration increasesrapidly. Compared with B(OH)−4 , OH− concentration is stilllower. So the adsorbed boron increases rapidly. When thepH further increases, OH− concentration will increase evi-dently, and the boron contribution decreases because of thecompetition for adsorption site with B(OH)−

4 .In this study, when the pH is lower than 10,Kd increases as

the pH increases (Fig. 2b). TheKd reaches its highest valuesat pH 10. When the pH is higher than 10,Kd decreases andshows a mild trend after pH 12. The variation characteristicsof Kd with the pH can also be explained partly by this model.But isotopic fractionation characteristics of boron, the max-imum adsorption value andKd value of brucite deposit cannot be explained by this totally.

The incorporation of B(OH)−4 into brucite may be relatedto adsorption. Previous studies (Liu et al., 2004; Garcia-Sotoand Camacho, 2006; Yuan et al., 2006) have indicated thatthe incorporation of boron into brucite or MgO was an ad-sorption process. When Mg2+ exists in solution, it can be ad-sorbed by brucite crystallite, and the resulting positive chargecan adsorb the negatively charged B(OH)−

4 . The equation isshown as follows:

{[Mg(OH)2

]m · n Mg2+

}2n+

+ xB(OH)−4 (5)

→ {[Mg(OH)2

]· n Mg2+

· xB(OH)−4 }2n−x

↓

If model 1 is applied, theδ11Bd should be lower thanδ11Bisw and αd−sw should be lower than 1. In this study,however, the results show that allδ11Bd are higher than



Fig. 6. The fraction (a) and δ11B (b) of boron species calculated using different pKa versus pH values. The influence of pKa on fraction and δ11B of boron species is big for solution with pH<10. When pH>10, the influence of pKa is insignificant. For example, the δ11BBisw3 calculated using pKa 7.0, 8.0 and 9.0 (α4/3=0.975) are 16.08‰, 5.57‰ and -4.74‰, respectively for pH 8, and 18.44‰, 18.21‰ and 15.89‰, respectively for pH 10. When pH >11, there are basically no differences among them (Fig. 6b).

24

Fig. 6. The fraction(a) andδ11B (b) of boron species calculated using differentpKa versus pH values. The influence ofpKa on fractionandδ11B of boron species is big for a solution with the pH< 10. When the pH> 10, the influence ofpKa is insignificant. For example,the δ11Bisw3 calculated usingpKa 7.0, 8.0 and 9.0 (α4/3 = 0.975) are 16.08 ‰, 5.57 ‰ and−4.74 ‰, respectively for pH 8 and 18.44 ‰,18.21 ‰ and 15.89 ‰, respectively, for pH 10. When the pH> 11, there are basically no differences among them (Fig. 6b).

δ11Bisw andαd−sw values are higher than 1. This suggeststhat model 1 is not suitable to explain all the data in our study.

The apparent pH dependence of the fractionation betweencarbonates and clays can be described in terms of theα3−4and thepKa of the boric acid-borate equilibrium (Sanchez-Valle et al., 2005). This model explains the B isotopic frac-tionation data for inorganically and biologically precipitatedcarbonates and clays, and suggests that in minerals like clayand other layered hydroxides, surface charge properties playan important role by changing the apparent acid-base equi-librium constant.

In our study,pKa of the boric acid-borate equilibrium maybe affected due to the presence of a positive charge on theMg(OH)2 surface. The fraction andδ11B of boron speciescalculated using differentpKa versus pH values is shown inFig. 6, which suggests that the influence ofpKa on frac-tion and δ11B of boron species is big for a solution withthe pH< 10. When the pH> 10, the influence ofpKa isinsignificant. For example, theδ11Bisw3 calculated usingpKa 7.0, 8.0 and 9.0 (α4/3 = 0.975) are 16.08 ‰, 5.57 ‰ and−4.74 ‰, respectively for pH 8, and 18.44 ‰, 18.21 ‰ and15.89 ‰, respectively for pH 10. When the pH> 11, thereare basically no differences among them. The pH values ofour experiments are higher 10, so the isotopic fractionationcharacteristics of boron are not attributed to the change ofpKa.

4.2 Model 2: chemical reaction of B(OH)3 withMg(OH)2

Rodionov et al. (1991) reported that when limewater wasadded to seawater, CaO· B2O3 · nH2O was deposited. Thisreaction is shown as follows:

2 H3BO3 + Ca(OH)2 + aq → CaO · B2 O3 · nH2O + aq (6)

During deposition of brucite, a similar reaction may occur.MgO · B2O3 · nH2O may also be deposited. This hypothesiswas confirmed by the XRD analyses of brucite. Comparedwith Fig. 7b, the brucite peak in Fig. 8 is clear, with ap-parent peaks of pinnoite (Mg(BO2)2 · 3H2O) and szaibelyite(MgBO2(OH)). Judging from the quantity and intensity ofthese peaks, pinnoite content should be higher than that ofszaibelyite. As the pH increases, the borate peak graduallyweakens (Fig. 7c–h). When the pH is 12.5 (Fig. 7h), its peakis very close to that of Mg(OH)2, though the characteristicpinnoite peak is still observed. The pinnoite is the result ofthe following reaction between brucite and H3BO3:

2 H3 BO3 + Mg(OH)2 → Mg(BO2)2 · 3 H2O + H2O (7)

The above reaction cannot occur for B(OH)−

4 . Thus,Fd3can exceedFfsw3 due to the preferential incorporation ofH3BO3 into Mg(OH)2, which enriches the deposited brucitein 11B. The presence of szaibelyite may be the result ofB(OH)−4 adsorption.

If model 2 is applied, theδ11Bd should be higher thanδ11Biswandαd−sw values should be higher than 1. Further-more,δ11Bd andαd−sw should decrease with increasing pHvalues. In our study, all theδ11Bd values are enriched in theheavy isotope compared relative to theδ11Bfsw (Fig. 3a), and



0 10000 20000 30000 40000 50000 60000 70000

2 Theta (deg)

0

200

400

600

800

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1200

Inte

nsity

(cps

)

(h) pH12.5

0 10000 20000 30000 40000 50000 60000 70000

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nsity

(cps

)

(f) pH11.0

0 10000 20000 30000 40000 50000 60000 70000

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nsity

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)

(g) pH11.5

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nsity

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)

(a) 70239 Mg(OH)2

0 10000 20000 30000 40000 50000 60000 70000

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nsity

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)

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0 10000 20000 30000 40000 50000 60000 70000

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nsity

(cps

)

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0 10000 20000 30000 40000 50000 60000 70000

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nsity

(cps

)

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2 Theta (deg)

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Inte

nsity

(cps

)

(e) pH10.5

Fig. 7. X-Ray diffractogram of brucite deposited from artificial seawater at different pH values. (a) XRD of brucite standard, (b) XRD of brucite from boron-free artificial seawater at pH 10.0, (c-h) XRD of brucite in our experiments. The brucite peak in our samples is clear compared with brucite standard (Fig. 7a).

25

Fig. 7. X-Ray diffractogram of brucite deposited from artificial seawater at different pH values.(a) XRD of brucite standard,(b) XRD ofbrucite from boron-free artificial seawater at pH 10.0,(c)–(h) XRD of brucite in our experiments. The brucite peak in our samples is clearcompared with the brucite standard(a).

all the αd−fsw are greater than 1 (Fig. 3b), which are con-sistent with model 2. But theαd−fsw shows a waved pattern(Fig. 3b), which indicates a complex isotopic fractionationprocess and cannot be explained completely by model 2.

The fraction of boric acid in solution is negligible athigh pH values (Fig. 1a), so that the amount of B(OH)3

incorporated into brucite should be little. But theFd3 is muchhigher than theFisw3 (Fig. 5). This may be related to thetransformation of borate to boric acid. After B(OH)3 is in-corporated into brucite,Fisw3 decreases. In order to keepthe balance of the B(OH)3/B(OH)−4 ratio, B(OH)−4 in solu-tion would transform to B(OH)3. Meanwhile, the total boron



100

200

20.000 40.000 60.0000

300400500

)spc(ytisnetnI

atadkaeP

kaepdra

C

07-0239 Mg(OH)2 Brucite. syn

PinnoiteSzaibelyite-M

I

IC

25-1119 Mg(BO2)2.3H2O33-0859 MgBO2(OH)

Fig. 8. X-Ray diffractogram of brucite deposited from B-containing artificial seawater at pH 9.5. It is a magnified figure of Fig. 7c. Compared with Fig. 7b, the brucite peak in Fig. 8 is clear, with apparent peaks of pinnoite (Mg(BO2)2·3H2O) and szaibelyite (MgBO2(OH)). Judged from quantity and intensity of these peaks, pinnoite content should be higher than that of szaibelyite. As pH increases, the borate peak gradually weakens (Fig. 7c-h). When pH is 12.5 (Fig. 7h), its peak is very close to that of Mg(OH)2 standard (Fig. 7a), though the characteristic pinnoite peak is still observed.

26

Fig. 8. X-Ray diffractogram of brucite deposited from B-containing artificial seawater at pH 9.5. It is a magnified figure of Fig. 7c. Comparedwith Fig. 7b, the brucite peak in Fig. 8 is clear, with apparent peaks of pinnoite (Mg(BO2)2 · 3H2O) and szaibelyite (MgBO2(OH)). Judgingfrom the quantity and intensity of these peaks, pinnoite content should be higher than that of szaibelyite. As the pH increases, the boratepeak gradually weakens (Fig. 7c–h). When the pH is 12.5 (Fig. 7h), its peak is very close to that of Mg(OH)2 standard (Fig. 7a), althoughthe characteristic pinnoite peak is still observed.

quantity in solution decreases compared with initial solution.When a new equilibrium is reached, B(OH)3 continues toincorporate into brucite, and B(OH)−

4 in solution will trans-form to B(OH)3 to keep the equilibrium. This process willrepeat, and finally theFd3 is much higher than theFisw3.However, the total boron quantity in solution is much lessthan that in the initial solution.

4.3 Model 3: both chemical reaction and adsorption ofboron incorporated into brucite

Many studies have indicated that only B(OH)−

4 is incorpo-rated preferentially into corals marine carbonates (Vengoshet al., 1991; Hemming and Hanson, 1992; Hemming et al.,1995; Spivack et al., 1993; Sanyal et al., 1996, 2000). Un-der this circumstance, all theδ11B values of carbonates areconsidered to be paralleled with the theoreticalδ11Bsw3 andare lower than that of the parent seawater;αcarb−sw betweencarbonates and seawater are lower than 1 and increase withpH. When boron is adsorbed by oxides or clay minerals, theadsorption ability of B(OH)−4 is much stronger than that ofB(OH)3. The B(OH)−4 is also incorporated preferentiallyinto oxides or clay minerals, causing the enrichment of10Bin oxides or clay minerals. Our results show thatδ11Bd ishigher thanδ11Bisw and parallels neither withδ11Bisw4 nor

with δ11Bisw3, but changes from decrease to increase with aninflexion at pH 12 (Fig. 3b). This indicates that both B(OH)−

4and B(OH)3 are incorporated into brucite, and the latter isincorporated into brucite preferentially. Boron incorporationinto brucite may be controlled by two processes: chemical re-action of B(OH)3 with brucite and B(OH)−4 adsorption ontobrucite. The simultaneous occurrence of these two processesdecides the boron concentration and isotopic fractionation ofbrucite. The mechanisms of boron incorporated into bruciteare distinct from those of carbonates and oxides or clay min-erals.

5 Judgment method for the existence of brucite incorals and its potentialapplication inδ11B-pH proxyand Mg/Ca-SST proxy

The incorporation of Mg into coral skeleton was controlledby varying factors. Fallon et al. (1999) suggested that vari-ations in Mg data inPoritescould be a result, not of tem-perature changes but of possible micro-scale heterogeneities.Mitsuguchi et al. (2003) reported an Mg/Ca offset and be-lieved to be a result of a biological/metabolic effect. Wein-bauer et al. (2000) studied the potential use of Mg as an envi-ronmental indicator in the coralCorallium rubrum, founding



60 70 80 90 100 110 120

Mg/Ca (mmol/mol)

18

21

24

27

30

33

36

39

42

δ11B

(0/

00)

60 70 80 90 100 110 120

Mg/Ca (mmol/mol)

7.4

7.6

7.8

8.0

8.2

8.4

8.6

pH

(a) δ11B = 0.01*(Mg/Ca)2 - 1.05*Mg/Ca + 56.4 R2 = 0.98

(b) pH = 0.0002* (Mg/Ca)2 - 0.0233* Mg/Ca + 8.14 R2 = 0.96

2 3 4 5 6 7

Mg/Ca (mmol/mol)

18

20

22

24

26

28

δ11B

(0/0

0)

(c) δ11B = -0.711*Mg/Ca + 25.3 R2=0.09

Fig. 9. Relationship (a) between Mg/Ca and δ11B in inorganic calcite, (b) between Mg/Ca and seawater pH (Xiao et al., 2006a), and (c) between Mg/Ca and δ11B in corals in Sanya Bay (Xiao et al., 2006b). The Mg/Ca was positive with δ11B in inorganic calcite and increased with seawater pH.

27

Fig. 9. Relationship(a) between Mg/Ca andδ11B in inorganic calcite,(b) between Mg/Ca and seawater pH (Xiao et al., 2006a), and (c)between Mg/Ca andδ11B in corals in Sanya Bay (Xiao et al., 2006b). The Mg/Ca was positive withδ11B in inorganic calcite and increasedwith the seawater pH.

that overall Mg incorporation was controlled by temperature.But the physiology within the coral colony may account fordiffering amounts of Mg among skeletal structures. A re-cent study (Nothdurft et al., 2005) showed that brucite existsin a wide range of common reef-building coral in Great Bar-rier Reef and Florida. Elevated Mg concentrations in modernscleractiniansmay promote the formation of high-Mg calcitecements, as observed in Holocene corals from Heron Reef(Nothdurft et al., 2005), and then cause the deviation fromthe normal SST-Mg/Ca curves. Samples of brucite-bearingcorals could be responsible for anomalies Mg/Ca vs. SSTplots in corals (Nothdurft et al., 2005). Thus, corals shouldbe cautiously used, especially brucite-bearing corals, to re-construct SST. If the variation of Mg/Ca can not be definedexactly, the Mg/Ca-SST proxy would become more complex.In addition, the influence of brucite in corals on theδ11B-pHproxy is more severe. Because one of the essential hypothe-ses for theδ11B-pH proxy is that only the B(OH)−4 incorpo-rated into corals, that is10B incorporated into corals prefer-entially (Vengosh et al., 1991; Hemming and Hanson, 1992;Sanyal et al., 1996). Under this circumstance, the boron iso-tope fractionation factor between coral and seawater is lessthan one. When brucite deposited from seawater,11B en-riched into brucite, the fractionation factor between bruciteand seawater is greater than one, which is entirely differentfrom those into corals (Fig. 3). The occurrence of brucite incoral can change the isotopic composition of boron in coraldue to the heavier B(OH)3 species incorporated preferen-tially into brucite and the deviant high boron isotopic compo-sition of corals may associate with the occurrence of brucite.If the brucite-bearing corals were used inδ11B-pH proxy, themeasuredδ11B of coral and the calculated pH will be higherthan the normal value. Rollion-Bard et al. (2011) suggestedthat careful sampling is necessary before performing boronisotopic measurements in deep-sea corals. So the existence

of brucite in corals can negatively influence the Mg/Ca-SSTproxy and theδ11B-pH proxy. How to judge the variationof δ11B and Mg/Ca in corals caused by brucite is importantfor the δ11B-pH proxy and the Mg/Ca-SST proxy. Abnor-mal boron isotope fractionation (αcarb/sw> 1) was observedin an inorganic calcite precipitation experiment carried outby Xiao et al. (2006a). They reasoned that the simultaneousdeposition of brucite with inorganic calcite was the main rea-son for this observation. Under this circumstance, theδ11Band Mg/Ca of inorganic calcite have a good positive relation-ship with correlation coefficients of 0.98 (Fig. 9a). In addi-tion, Mg/Ca was independent of SST, but increased with sea-water pH (Fig. 9b), indicating the high Mg/Ca ratio andδ11Bwere due to the increasing brucite deposition as seawater pHincreases. These observations provide a new method for dif-ferentiating the existence of brucite in corals. The weak neg-ative relationship between theδ11B and Mg/Ca in corals inSanya Bay (Fig. 9b) indicated there is no brucite existing incorals in this area. Thus, the relationship betweenδ11B andMg/Ca in corals can be used to judge the existence of brucitein corals, which should provide a reliable method for betteruse ofδ11B and Mg/Ca in corals to reconstruct paleo-marineenvironment.

6 Conclusions

Based on the above experimental results, the following con-clusions can be drawn:

1. Boron concentration of deposited brucite and partitioncoefficientKd between deposited brucite and resultantseawater is controlled by the pH of solution. The incor-poration capacity of boron into brucite is much strongerthan that into oxides and clay minerals.



2. All the δ11Bd values are higher thanδ11Bfsw and all thefractionation factorsαd−fsw are higher than 1, indicatingthe preferential incorporation of H3BO3 into Mg(OH)2.The mechanism of boron incorporated into brucite isdistinct from that of carbonates deposition.

3. During deposition of brucite, both H3BO3 and B(OH)−4incorporated into brucite. The simultaneous occurrenceof boron adsorption onto brucite and the precipitationreaction of H3BO3 with brucite decide the boron con-centration and isotopic fractionation of the resultingbrucite.

4. The existence of brucite in corals can affected theδ11Band Mg/Ca in corals and influences the Mg/Ca-SSTproxy andδ11B-pH proxy negatively. The relationshipbetweenδ11B and Mg/Ca in corals can be used to judgethe existence of brucite in corals, which should providea reliable method for better using ofδ1111B and Mg/Cain corals for the reconstruction of the paleo-marine en-vironment.

Acknowledgements.This project was supported by the NationalNatural Science Foundation of China (Grant No. 40573013,40776071 and 41003012), and by the innovation fund of the Insti-tute of Earth Environment, Chinese Academy of Sciences (GrantNo. 0951071293). The authors thank Mr. Guohong Gong, Bo Yangand Hua Ge for conducting the XRD and SEM analyses. We alsothank the editor Christine Hatte and two anonymous reviewersfor their constructive comments and assistance in improving themanuscript.

Edited by: C. Hatte

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